In ethylene slurry polymerization processes, diluents such as hexane are used to dissolve the ethylene monomer, comonomers, and hydrogen. The monomer(s) are polymerized with a catalyst.
In some multi-reactor cascade processes, monomer(s), hydrogen, catalyst and diluent are fed into a first reactor where a slurry forms from the polymer particles contained within the diluent and unreacted monomer. The reactors can be operated in parallel or in series, and the types/amounts of monomer and conditions can be varied in each reactor to produce a variety of polyethylene materials, including unimodal (molecular weight distribution) or multimodal polyethylene material.
Ziegler type catalysts have been used in ethylene polymerization processes. These catalysts use aluminum alkyl compounds as co-catalyst activators to activate titanium or vanadium sites on the catalyst. The amount of co-catalyst present in the reactor affects the yields and selectivity of the ethylene slurry polymerization process.
Various compounds, such as oxygen-containing polar molecules, can poison Ziegler type catalysts, thereby degrading yields and selectivity. These compounds can be contained as impurities in various feeds to the polymerization such as in the monomer(s), in the diluent, or in other feed streams.
A need exists for multi-reactor ethylene slurry polymerization processes in the production of multimodal polyethylenes to (i) minimize the adverse effects of catalyst poisons on reactor yields and selectivity and (ii) ensure that polyethylenes of constant structure and composition are produced in each polymerization reactor.